1. Field of the Invention
The present invention relates to a method of suppressing galloping of multiconductor transmission lines.
2. Description of the Related Art
If ice forms in a wing-shaped manner on the upwind side of an overhead transmission line, a wind from substantially horizontal direction will create a lift, which will cause the line to vibrate vertically and induce "galloping" of a self-oscillation of about 0.1 Hz to 1 Hz. When the wind is strong, this galloping causes the line to vibrate with mainly a large vertical motion with some horizontal motion. The maximum vertical amplitude of this vertical motion becomes as large as 10 meters. This large vertical motion of the lines sometimes causes lines above and below each other to come into contact and therefore causes short-circuits between them.
This galloping will be explained with reference to FIG. 1, which is a vertical sectional view of a transmission line in the line direction (longitudinal direction). In FIG. 1, if ice 4 forms in a wing-shaped manner on the upwind side of a conductor 2 of an overhead transmission line at the position .alpha. and this is struck by a horizontal wind, the wind causes a lift at the wing of ice 4 and the conductor 2 rises to the position .beta.. When rising, the conductor 2 is twisted in the clockwise direction and the ice 4 turns upward as shown at the position .beta.. At the position .beta., a further lift is caused by the wind at the upward turning ice 4 formed on the conductor 2 and so the conductor 2 rises to the position .gamma.. After rising to the position .gamma. in the figure, the conductor 2 descends to the position .delta. due to the elasticity of the conductor. At this time, the conductor is twisted in the counterclockwise direction, the ice 4 turns downward as shown at the position .delta., the conductor 4 with the downward turning ice 4 descends to the lowest limit position .epsilon. due to the wind, then once again rises. As a result, the conductor 2 engages in repeated torsion and vertical motion. As mentioned above, the line engages in a torsional vibration wherein a further upward force acts on the line when rising and a further downward force acts on it when descending. Accordingly, the vertical motion of the conductor 2 develops into large galloping.
This galloping occurs more easily in a multiconductor transmission line than a single conductor transmission line. In the case of four conductors, for example, spacers 6 are attached between the four conductors 2a to 2d as shown in FIG. 2, which is a vertical sectional view in the direction of the transmission line. At the position a in FIG. 2, when ice 4 forms in a wing-shaped manner on the upwind sides of the conductors 2a, 2b, 2c, and 2d of the four-conductor transmission line 2A and these are struck by a horizontal wind, the wind causes a lift which causes the transmission line 2A to rise and twist in the clockwise direction. Accordingly, the wings of ice 4 turn upward, the lift caused by the wind increases, and the transmission line 2A rises from the position a to the position .beta.. Next, the four-conductor transmission line 2A descends, the conductors 2a to 2d are twisted in the counterclockwise direction, and the wings of ice 4 turn downward, whereupon the wind causes a downward force and the line descends from the position .beta. to the positions .gamma. and .delta.. Next, the line rises from the position .delta. to the positions .alpha. and .beta.. That is, the four-conductor transmission line 2A engages in repeated torsion and vertical vibration. As mentioned earlier, the line engages in torsional vibration wherein a further upward force acts on the line when rising and a further downward force acts on it when descending. Accordingly, the vertical motion of the transmission line 2A develops into a large galloping.
FIG. 3 shows the results of measurement of the vertical displacement, horizontal displacement, and torsional displacement of conductors caused by such galloping in the case of a four-conductor transmission line with a sectional area of conductors of 810 mm.sup.2. The torsional vibration and the vertical vibration match in vibration periods, but are slightly deviated in phase.
To prevent this galloping, it is possible to attach anti-vibration dampers to the line, but dampers are not sufficient by themselves to prevent large galloping of a multiconductor transmission line. Therefore, to prevent short-circuits between a top phase line and bottom phase line due to galloping, interphase spacers made of insulating materials, such as ceramic insulators, are attached between the top phase line and bottom phase line. For example, as shown in FIG. 4, in the case of a double-conductor transmission line, an interphase spacer 10 made of an insulating material is attached between the spacer 8a of the top two conductors 2B, 2B and the spacer 8b of the bottom two conductors 2C, 2C.
As the insulating material forming such a conventional interphase spacer 10, in general use is made of a ceramic insulator. Since an interphase spacer 10 is long in length, it is necessary that the ceramic insulator spacer 10 not break when subjected to the compressive load from the two lines at the two ends of the spacer 10. Therefore, the spacer 10 has to be made thick in diameter. If thick interphase spacers 10 are attached, however, the weight of the ceramic insulator interphase spacers 10 attached to the transmission lines as a whole becomes greater, which invites an increase in the tension on the lines and an increase in the strain of the lines at the point of attachment of the interphase spacers. The steel towers therefore become insufficient in strength and require reinforcement and therefore extra trouble is entailed. Accordingly, use has been made of plastic interphase spacers with small weights rather than ceramic insulator spacers 10, but the weight of the interphase spacers as a whole has still not sufficiently been reduced.
The galloping causes the lines to twist and adds to the vertical motion. If the torsional vibration and the vertical vibration match in frequency, they develop into galloping of a large amplitude. Accordingly, it was not possible in the past to effectively prevent the occurrence of large amplitude galloping even if dampers were provided to prevent twisting of the lines. That is, in the related art, it was not possible to effectively prevent torsional vibration causing large galloping aggravating the vertical vibration of the lines.